Implement Knocking on Element Web

[jakewb-b]

Links

• PRD (internal): https://docs.google.com/document/d/15ornTrYI5UYVkvlHcD89IIhYIKvdfZcPn6V-KVnMmx8/edit
• Spec: https://spec.matrix.org/v1.2/client-server-api/#knocking-on-rooms
• Figma designs: https://www.figma.com/file/7TqjqdMBaPpm3IavKsMYu6/%5BBWI%5D-Knocking?type=design&node-id=367-50166&mode=design&t=6Bjqu0KINaAjkbV0-0
• Screenshots of proposed design: Implement Knocking on Element Web #18655 (comment) (superceded by Figma??)
• Already implemented by Synapse, just need UI

Mobile (out of scope for this issue): See element-hq/element-android#3875 and element-hq/element-ios#4731

### Tasks
- [ ] https://github.com/vector-im/element-web/issues/26043
- [ ] https://github.com/vector-im/element-web/issues/26044
- [ ] https://github.com/vector-im/element-web/issues/26083
- [ ] https://github.com/vector-im/element-web/issues/26084
- [ ] https://github.com/matrix-org/synapse/issues/16307
- [ ] https://github.com/element-hq/element-web/issues/27025

[Mikaela]

I am unable to open either of the links from the original submission and am wondering whether it's noted that this is part of Matrix protocol specification version 1.1?

• https://spec.matrix.org/latest/client-server-api/#knocking-on-rooms